A PDP (plasma display panel) uses a matrix array of discharge cells, which can only be “ON”, or “OFF”. Also unlike a CRT (cathode ray tube) or LCD (liquid crystal display) in which gray levels are expressed by analog control of the light emission, a PDP controls the gray level by modulating the number of light pulses per frame (sustain pulses). This time-modulation will be integrated by the eye over a period corresponding to the eye time response. Since the video amplitude is portrayed by the number of light pulses, occurring at a given frequency, more amplitude means more light pulses and thus more “ON” time. For this reason, this kind of modulation is also known as PWM, pulse width modulation.
For all displays using pulse width modulation, the number of real gray levels is limited. For PDP, in case of standard coding the number of gray levels is more or less equal to 256.
These various gray levels can only be used when the dynamic of the input picture is at its maximum (in case of 8 bit signal, video values between 0 and 255). In other cases, when the dynamic is reduced (in particular because of contrast or brightness parameters), the number of displayed levels will further decrease.
The problem is that the picture quality is affected when the number of displayed levels is reduced.
Unfortunately, when reducing the contrast (by dividing by a certain factor) and/or the brightness (subtracting a certain coefficient from the picture), the maximum value of the picture decreases and so the picture quality is reduced.
Contrast and brightness controls are usually part of the so called “front-end”, while PDP specific functions (gamma function, Sub-field encoding, etc) are part of the so called “back-end” of the display (see FIG. 3).
In the back-end of a PDP an APL function is used to control the power. The computation of this Average Power Level (APL) is made through the following function:
      A    ⁢                  ⁢    P    ⁢                  ⁢          L      ⁡              (                  I          ⁡                      (                          x              ,              y                        )                          )              =            1              C        ×        L              ·                  ∑                  x          ,          y                                              ⁢              I        ⁡                  (                      x            ,            y                    )                    where I(x,y) represents the picture to display, C the number of columns and L the number of lines of the PDP.
The aim of power management is to keep the power consumption constant (see FIG. 1) and to have a peak luminance as high as possible. So for every APL value, the maximal number of sustain pulses to be used is fixed. This number of sustain pulses decreases when the APL increases, and vice versa as shown in FIG. 2.
In peak-white pictures (low APL at the left side of FIG. 2), the number of sustain pulses is not limited by the power consumption, but by the available time for sustaining. For this reason, the power consumption of peak-white pictures will be lower than for the other pictures. Consequently, also the power consumption decreases for low APL levels (compare FIG. 1).
The following table shows an allocation of the values of the number of sustain pulses to the average power levels according to FIG. 2.
Total Number ofAPLsustain pulses010001100021000310004100051000...5010005110005210005310005410005510005699957998589965999460991619886298463979649756597166966679626895869954709507194672942739387493375929...295449296448297447298446299445300444301442302441303440304439305438......600225601224602223603223604222605222606221607221608221609220610220611219612219613219614218615218...1005102100610210071021008102100910210101021011101101210110131011014101101510110161011017100101810010191001020100102110010221001023100
As indicated above, the problem of the standard implementation of power management is that when the energy of the input picture of the back-end decreases, the number of sustain pulses increases. So the energy of the displayed picture decreases hardly.
FIG. 3 shows a principle block diagram of the driving unit of a plasma panel 1. The video input signal is first processed in the front-end 2. The front-end includes a scaling unit 4 for adapting the size of the picture to that of the panel. The scaled input signal is supplied to a brightness/contrast control block 5. This control block 5 receives external signals for tuning the brightness and/or the contrast of the picture (=adjusting the gain and/or the offset of the video data). The video signal is processed accordingly and supplied to the back-end 3. Within the back-end 3 the signal is processed in a usual path including a gamma block 6, a dithering block 7 and an encoding block 8. The gamma block 6 performs a data transformation with a look up table in accordance to a nearly quadratic gamma function. The output signal of the gamma block 6 is transmitted to the dithering unit 7 which will add for example 4 bit dithering in order to render more discrete video levels at the output. Afterwards, the sub field encoding 8 generates sub field data for the video signal. The resulting sub field data are sent to the plasma panel 1.
In a parallel path within the back-end 3 the output signal of the front-end 2 is input into an APL measurement block 10. This block supplies an APL level of the brightness/contrast tuned video signal to the power management 9. The power management 9 controls the gamma unit 6 and the encoding unit 8. Furthermore, the power management 9 delivers sustain information to the plasma panel 1.
With this arrangement, it is for example interesting to see what happens when the user is decreasing the contrast and/or the brightness.
When decreasing the contrast and/or the brightness, the APL (measured in the back-end 3) is decreasing; this means that the number of sustain pulses is increasing. This increases partly the contrast.
For example, the user wants to reduce the contrast by 2 for a picture, which has an APL of 300 (10 bit value). So originally this picture has in average approximately 444*300/1024=130 sustain pulses/cell, and can have a peak luminance of 444 sustain pulses (compare table shown above).
To obtain in average 65 sustain pulses/cell, the user in fact has to reduce the contrast of the picture by around 4. (for an APL value of 70, according to the table, the average number of sustain is equal to 950*70/1024=65). The peak luminance in this case is also reduced since all brightness levels of the whole picture are divided by more than 4, the maximum value of the picture will not be higher than 255/4.3=60 (this represents 950/4.3=222 sustain pulses). But since, the picture is divided by more than 4, the number of gray levels really used is also divided by around 4. The picture quality is rather low in this case.